1. Field of the Invention
The present invention relates to a developer unit for use in image forming apparatus such as an electrophotographic copying machine, printer, or the like.
2. Description of the Prior Art
Two types of developer are known and widely used for effecting magnetic brush development of electrostatic images, one being a two-component developer consisting of a mixture of magnetic toner and magnetic carrier and the other being a one-component developer consisting only of magnetic toner.
The two-component developer requires that the magnetic carrier and toner be rubbed each other to charge toner particles, and this is difficult to achieve a good quality image unless the mixing ratio of the magnetic carrier to the toner is maintained within a prescribed range. Another difficulty is that the toner or toner component adheres to and accumulates on the surface of the magnetic carrier over a long period of use (such toner being generally called "spent toner"), thereby making it difficult to charge the toner.
The one-component developer has the advantage that it can be used for development of images without the use of a magnetic carrier, but the disadvantage is that the bristling of the magnetic brush substantially varies with the environmental changes such as temperature and humidity, thereby making it difficult to maintain a constant amount of bristling.
To overcome the problems pointed out above, Japanese Laid-Open Patent Publication No. 59-75269 (Prior Art 1) discloses a 1.5-component which consists of a bristling accelerating component composed of magnetic particles and a magnetic toner component composed of particles of resin/magnetic powder dispersion.
Since it is composed of a single kind of magnetic substance such as ferrite, the bristling accelerating component has superior magnetic characteristics to that of the magnetic toner component composed of magnetic powder dispersed in resin powder, and therefore improves the bristling of the magnetic brush on the sleeve.
In addition, when the bristling accelerating component is used in combination with the magnetic toner, the bristling of the magnetic brush is always maintained in good condition regardless of changes in the environment of the developer, the concomitant advantage being such that the mixing and stirring effect is given to the developer while it is being carried on the sleeve, serving to maintain the developer component in the powder state with good flowability regardless of changes in the environment such as temperature or the humidity.
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 63-85663 (Prior Art 2), a magnetic roll is rotated, while the rotation of the developing sleeve is stopped, during the time of operation other than recording, so that the developer bristles formed on the developer sleeve stand up high and a large amount of developer flows from the blade. This helps to reduce the pressure applied to the developer in the area surrounded by the developing sleeve, the blade, and the net-like member, and facilitates remixing with the developer which has been being mixed and stirred in that area, thereby assuring preparation of a uniform mixture.
In the developing method of Prior Art 1 using the 1.5-component developer, the toner is charged due to the movement of the developer on the sleeve. However, the flow of the developer is limited by the doctor, etc., and the amount of the developer carried on the sleeve decreases, with a resultant drop in the ability of the sleeve to stir the developer. The problem is that when this stirring ability drops, the toner shows a marked drop in its chargeability, with a resultant drop in the image density.
In Prior Art 2, when the pressure is reduced on the developer in the area surrounded by the developing sleeve, the blade, and the net-like member, a large amount of developer flows from the blade to reduce the pressure so that the toner density that has been decreased can be restored, but the charge amount cannot be restored.
Another problem with Prior Art 2 is that when the construction is such that the pressure increases on the developer on the upstream side of the blade, the toner density cannot be restored because the pressure is not reduced.